In verse 15 there were parents bringing their infants to Jesus. The disciples rebuke them. Jesus rejects this rebuke and encourages the parents to bring their children to him. Then, as if to warn the disciples, he says, “Whoever does not receive the kingdom of God like a child shall not enter it.” In other words, when Jesus sees a disciple hindering a child from coming to him, he sees someone who is in danger of missing the kingdom because of pride.
If you are receiving the kingdom yourself like a little child, then you will not do anything to hinder little children from coming to Jesus. But if you are trying to enter the kingdom some other way than by receiving it like a child, then you will probably be a hindrance to children. If you are not childlike toward God, children will probably be beneath you and not worth your time.
So there is a very close connection between your own humility and your ability to lead children to Jesus. The great hindrance to effective ministry to children is pride, and the great gift for ministry to children is humility.